Martin et al., in U.S. Pat. No. 4,228,050, have disclosed an improved conductive molding composition for the compression molding of high density information discs, also known as video discs. This composition, in essence, comprises a polyvinylchloride-based resin containing sufficient conductive carbon black particles to provide the requisite conductivity, i.e., a bulk resistivity of below about 500 ohm-cm at 900 mHz. The formulation additionally contains additives such as stabilizers, lubricants, plasticizers, processing aids and the like. The composition disclosed by Martin et al. contains a large number of such additives which are required to stabilize the finished disc. It has been found in developing this formulation that care must be used in selecting each additive because not only its own stability must be taken into account, but one must examine the stability of each additive vis-a-vis the other additives, their degradation products, their potential reactions with each other, as well as with or between each other. Obviously, the more ingredients that are present the more complex the problem becomes and potentially the greater the possibility that reactions will occur, for example, during the heat and pressure encountered in compression molding of the disc.
Even the complex composition disclosed by Martin et al. is inadequate to prevent deterioration of the disc upon exposure to conditions of elevated temperature and relative humidity. This deterioration affects the playback characteristics of the disc so that they are no longer excellent and maybe even progress to where they are unacceptable. In order to offset this deterioration, the discs, after they are pressed, are treated to remove any material on the surface of the disc. For example, they may be washed, with an aqueous cleaning solution containing an oxidizing agent, a base and a fluorosurfactant as disclosed in U.S. Pat. No. 4,275,100 to Datta, and then rinsed with water and dried. In addition to the treating steps, the discs are further coated with a methylalkylsiloxane lubricant which is doped with a diloxane dopant as disclosed in U.S. Pat. No. 4,355,062 to Wang et al. Discs thus washed and lubricated have been found to possess excellent playback characteristics over extended periods of time, even under conditions of high temperature and high relative humidity.
There exists, however, a need to simplify the formulation of the video disc molding composition thereby reducing the number of chemical reactions which can occur during and after manufacture and/or reduce the number of processing steps required for the disc's manufacture for an obvious savings in cost and time. In accordance with this invention, there is provided an improved conductive molding composition for the video disc which significantly reduces the number of ingredients required, eliminates the required further treating or washing operation and the use of doped lubricants, while retaining and even improving the playback performance characteristics of the finished discs.